1. Field of the Invention
The present invention relates generally to a control system for an automotive suspension system, which controls suspension characteristics depending upon vehicle driving conditions. More specifically, the invention relates to a suspension control system which utilizes information representative of a lateral acceleration exerted on the vehicular body as a vehicular driving condition representative parameter.
2. Description of the Background Art
In the modern automotive technologies, it has been considered an important task to achieve both riding comfort and driving stability by adjusting suspension characteristics of a vehicular suspension system. For example, Japanese Utility Model First (unexamined) Publication (Jikkal) Showa 62-38402 discloses a suspension control system which performs control of suspension characteristics depending upon vehicle speed and the magnitude of lateral acceleration exerted on the vehicular body. In the practical control as disclosed, the control system is responsive to vehicle speeds higher than or equal to 30 km/h and lower than 80 km/h and to lateral acceleration greater than or equal to 0.2 to harden suspension characteristics; and the control system is responsive to the lateral acceleration magnitude greater than or equal to 0.1 to harden suspension characteristics while the vehicle speed is higher than or equal to 80 km/h. With such process of suspension control, a certain level of improvement in riding comfort and driving stability can be achieved.
In such a prior proposed suspension control system, the suspension characteristics are determined simply depending upon the magnitude of lateral acceleration and irrespective of the mode of vibration induced in the vehicular body. As will be appreciated, vehicular body vibration is caused by various causes, and the mode of vibration is variable depending upon the causes. Namely, when the vehicular body rolls during cornering, the vehicular vibration is in a rolling mode with relatively low frequency and relatively great magnitude. On the other hand, when the vehicular vibration is caused by undulation on the road surface or by road shock, vehicular body vibration is in a bounding and rebounding mode with relatively high frequency and relatively small magnitude. Depending upon the mode of vibration, required suspension characteristics at the same magnitude of lateral acceleration may be different. For example, in vehicular rolling, it is preferred to harden suspension characteristics in order to provide higher, cornering stability. On the other hand, on certain occasions, a magnitude of lateral acceleration equivalent to the vehicular rolling mode can be induced in a bouncing mode vibration when the vehicle passes over a projection on the road, such as a step at road side edging. In such a case, as set forth above, a harder suspension is preferred in response to vehicular rolling. On the other hand, in the case of bouncing, a softer suspension effective for absorbing vibration energy is preferred. Since the prior proposed suspension control systems cannot adapt the suspension characteristics to the mode of the vehicular body vibration, the control is not completely satisfactory as to the desired level of riding comfort and driving stability.
In addition, the prior proposed suspension control system switches suspension characteristics from softer characteristics to harder characteristics every time the magnitude of lateral acceleration exceeds a predetermined criterion irrespective of mode of vibration. Therefore, even when the vibration mode does not require harder suspension, a switching of suspension characteristics is caused. This necessitates an unnecessarily high frequency of switching operation for shorten life of suspension characteristics adjustable valve structure and/or actuator.